Interesting point. If true, because Tesla doesn't build in VIN order, it means someone may get a 2017 VIN built in 2016 because their order got pushed, and someone else could get a 2016 built in 2017 because their VIN was assigned too early or there was a parts shortage.
Someone on this forum just got their VIN within the last 2 days and it is a G (2016) VIN, even though it's only 5 weeks until 2016, and Tesla would need to build 7000+ Model X's in those 5 weeks to clear all the G VINs in 2016.
Of course, other manufacturers don't actually follow a Jan 1 cutoff either. You can often buy 2017 model year cars in 2016.